1. Field of the Invention
The present invention relates in general to ceramic compositions. In particular, the present invention is directed to ceramic compositions for producing multilayer ceramic inductors. The ceramic compositions disclosed herein can be sintered at relatively low temperatures to produce low-dielectric dense bodies.
2. Description of the Related Arts
One of major trends in electronic packaging industry is to use surface mount technology (SMT) as a replacement for conventional plated through-hole (PTH) technology. SMT offers several distinct advantages including greater packaging density, higher lead counts with shorter interconnection lengths and easier automation over PTH technology. Since SMT demands electronic devices and components to be mountable on the surface of a printing wiring board, the materials and structure of traditional leaded components including capacitors, resistors and inductors have to be redesigned to meet the requirements of short, thin, light and small electronic devices era. In the past 30-year development of surface mountable passive components, inductor is the least successful one, because it has wire to wind and is difficult to be miniature without degradation of magnetic properties. Magnetic shielding is also important, especially for high packaging density; therefore, the inductor has to be boxed to prevent magnetic flux leakage and cross talks between inductors. To fulfill the above requirements, the multilayer chip inductor (MLCI) is thus developed.
Two major technologies including dry and wet processes have been developed to fabricate MLCI. In the dry process, thin and flat ceramic green tapes are first made by tape casting, and then each green tape is via punched and screen printed with metal paste. The thus made green tapes are stacked, laminated and cofired to obtain the final monolithic MLCI parts. Except via punching step, the dry process is similar to that used in manufacturing multilayer ceramic capacitors (MLCC). For the wet process, the tape casting is used to prepare the first ceramic layer on which the multilayer internal winding structure is fabricated by screen printing. Metal paste is screen printed by a half turn winding on the ceramic green tape and then the ceramic paste is screen printed to cover the metal layer except the connection part of winding. The above alternating processes continue until the required number of winding layers is achieved. The multilayer structure is then covered and laminated with a ceramic green tape, and finally cofired to obtain MLCI parts. Metal as well as ceramic pastes are needed in this process.
For high-frequency MLCI applications, a low-dielectric-constant, low-temperature cofirable ceramics (LTCC) metallized with high-electrical-conductivity metals such as Ag has been developed. Most of these LTCC tape systems have dielectric constants between 5 and 10 and encompass a range of low dielectric loss between 0.01-0.5% at frequency greater than 300 MHz. This invention is to disclose a low temperature co-firable MLCI system using a glass plus ceramic approach that can be densified at 800-1000.degree. C. for 15-60 min. The densified LTCC system has a dielectric constant below 8.0 and a dielectric loss of 0.1-0.2% at 1 MHz.
There have been numerous attempts to make a low temperature co-firable MLCI substrate. A ferrite composition comprising ferrite, borosilicate glass, and optionally boron is described in U.S. Pat. No. 4,956,114. The ferrite composition can be fired at a relatively low temperature of 950.degree. C. into a sintered body having improved mechanical strength and electromagnetic properties. The sintered ferrite body is used to form a chip inductor.
U.S. Patent No. 4,540,500 discloses a magnetic ferrite material that can be sintered at a low temperature by adding a Li.sub.2 O--B.sub.2 O.sub.3 --SiO.sub.2 glass into the mother ferrite material.
A method for producing multilayer ceramic circuit boards for use with copper conductors is described in U.S. Pat. No. 4,642,148. It discloses a ceramic composition comprising 10-75% by weight of alpha-alumina, 5-70% by weight of non-crystalline quarts (fused silica), and 20-60% by weight of borosilicate glass. The sintered ceramic composition has a dielectric constant ranging from 4.8 to 9.6.
U.S. Pat. No. 4,672,152, issued to Shinohara et al., describes a multilayer ceramic circuit board in which a ceramic insulating layer is prepared from a mixture of 50-95% by weight of crystallized glass and 5-50% by weight of a ceramic filler. The crystallized glass consists of 5-20% by weight of lithium oxide, 60-90% by weight of silicon dioxide, 1-10% by weight of aluminum oxide, and 1-5% by weight of alkal metal oxide other than lithium oxide. The ceramic filler is selected from the group consisting of silicon dioxide, .beta.-eucryptite (LiAlSiO.sub.4), and aluminum oxide.
U.S. Pat. No. 3,926,648, issued to Miller et al., discloses a sintered glass-ceramic body containing hexagonal cordierite as the crystalline phase. The glass composition exhibits a dielectric constant of 5.2 and a coefficient of thermal expansion between 1-2.times.10.sup.-6 K.sup.-1.
U.S. Pat. No. 4,755,490, issued to DiLazzaro, describes a low firing temperature ceramic material having a dielectric constant between 4.5 and 6.1 and a coefficient of thermal expansion between 3.9-4.2.times.10.sup.-6 K.sup.-1. The ceramic material is provided from a mixture of 10-50% by weight of alumina, 0-30% by weight of fused silica, and 50-60% by weight of a frit composed of 4% by weight of CaO, 12% by weight of MgO, 29% by weight of B.sub.2 O.sub.3, and 42% by weight of SiO.sub.2. The mixture has a minimum sintering temperature in the range of 850-1000.degree. C.
U.S. Pat. No. 4,788,046, issued to Barringer et al., discloses glass-ceramic packages for integrated circuits by co-sintering a glass-ceramic composite and Cu, Ag, Pd, or Au at a low sintering temperature. The glass-ceramic composite is provided by coating glassy compounds onto ceramic particles. The composite has a low dielectric constant of 4.5 and a thermal expansion coefficient greater than 5.5.times.10.sup.-6 K.sup.-1.
U.S. Pat. No. 4,879,261, issued to Burn, discloses a low dielectric composition for making a ceramic dielectric body having a dielectric constant less than 5.0. The composition is a mixture of finely divided particles consisting essentially of 70-85% by weight of silica and 15-30% by weight of zinc borate flux. The composition can be used to make green tape and multilayer devices having internal copper conductors such as multilayer capacitors and multilayer interconnects.